Symbols observer.

Create an observer that generates events with symbol information.

When an Image::DCStart or Image::Load event is processed by ETWConsumer, SymbolsObserver tries to load the symbols of the image and generates the following events:

- An event to associate an identifier with the image.
- An event for each symbol of the image. This event contains the image identifier, symbol name and symbol address.

R=etienneb@chromium.org

Review URL: https://codereview.appspot.com/17760043